Glasses of the Na2 0--T2 05 --SiO2 and the Na2 0--L2 0--a2 05 --SiO2 system

ABSTRACT

Thermally crystallizable glasses of the Na 2  O--Ta 2  O 5  --SiO 2  and the Na 2  O--Li 2  O--Ta 2  O 5  --SiO 2  systems, and glass-ceramics made therefrom which are highly transparent, have high indices of refraction, and excellent strength properties. The glasses of the first system consist essentially of about 37-55 mole percent SiO 2 , 23-35 mole percent Ta 2  O 5 , and 20-33 mole percent Na 2  O, preferably where the molar ratio of Na 2  O to Ta 2  O 5  is about 1, and for the second system the glasses consist essentially of about 27-45 mole percent SiO 2 , 32-45 mole percent Ta 2  O 5 , and 20-35 mole percent Li 2  O plus Na 2  O, preferably wherein the molar ratio of Li 2  O to Na 2  O is within the range of from 0.7:1 to 1.2:1, and the molar ratio of Na 2  O + Li 2  O to Ta 2  O 5  is within the range of 0.5:1 to about 1:1. Glasses and glass-ceramics of the latter system also have good dielectric properties and low loss tangents. By varying the heat treatment schedule for crystallization of a glass to a glass-ceramic, a specific high index of refraction coming within a prescribed range may be imparted to the finished, transparent glass-ceramic. A transparent glass-ceramic having two or more different indices of refraction may also be produced.

With the advent of acoustooptical devices, electrooptical devices, andother devices utilizing transparent glasses and glass ceramics, the needfor glasses and glass-ceramics having a high index of refraction isbecoming more acute. However, it is well-known that the higher the indexof refraction of a glass, the more difficult it is to obtain it inoptical quality (freedom from bubbles, stones and cords). This isespecially true when the index of refraction is about 1.8 or higher.

Acoustooptical devices (sometimes called elastooptical devices) areknown in the art and reference is made to an article entitled "A Reviewof Acousto-optical Deflection and Modulation Devices" by E. I. Gordon,which appeared in the Proceedings of the I. E. E. E. [10] 1391-1401(1966) and an article entitled "Dielectric Materials for Electrooptic,Elastooptic and Ultrasonic Device Application" by E. G. Spencer et al.,appearing in the Proceedings of the I. E. E. E., 55 [ 12] 2074-2108(1967).

Electrooptical devices are also well-known and are described in U.S.Pat. No. 3,069,973 to I. Ames and in U.S. Pat. No. 3,467,463 to N. F.Borrelli et al.

While single crystal materials are of high quality and can be used inacoustooptical and electrooptical devices, they are relatively veryexpensive to obtain, especially in large sizes and moreover one islimited to the precise index of refraction inherent in any particularsingle crystal.

The present invention is directed to certain limited compositions havingspecific parameters and coming within the broad definition of alkali-Ta₂O₅ --SiO₂ systems which can be readily formed into glasses and theglasses subsequently thermally in situ crystallized to form transparentglass-ceramics having selected high indices of refraction. Suchglass-ceramics can be formed from the crystallizable glasses even thoughthe glasses do not have any of the customary nucleating agents, such asTiO₂, ZrO₂, P₂ O₅, and the like. Glasses and glass-ceramics of thisinvention are suitable for use in electrooptical devices, acoustoopticaldevices, and the like, for example as modulators, laser Q-switchesand/or deflectors, etc. Certain of such glasses and glass-ceramics havebeen found to also have good dielectric properties including very lowdielectric losses, which make them suitable for use in a variety ofelectrical devices, such as capacitors, electroluminescent cells, waveguides, and the like. Dielectric constants of at least 50 (at roomtemperature and 0.5 MHz) with loss tangents or dissipation factors ofless than 3% and preferably less than 1% are obtained withglass-ceramics of the present invention.

By use of the thermally crystallizable glasses of the present inventionhaving a constant composition, each glass can, during an appropriateheat treatment process, be converted to a transparent, inorganiccrystalline oxide ceramic material having any desired index ofrefraction between a selected upper and a selected lower limit.Furthermore, the ceramic material thus formed, also known as aglass-ceramic, exhibits a relatively high optical index of refractionand contains a major portion of very tiny crystals embedded in a minorglassy matrix remaining as a result of the thermal crystallization.

Furthermore, glass-ceramics of this invention can be prepared in theform of small chips, of a size of about one-fourth inch and less, whichare suitable for use as support carriers for electronic microcircuits,which chips have extremely high strengths and are difficult to breakeven after they have been first scribed with a diamond scribe. Thestrengths of such compositions are relatively high as compared tostrengths of glass-ceramic chips having a composition outside of thepresent invention and which can usually be broken readily with one'sfingers.

Within the broad spectrum of the alkali-Ta₂ O₅ --SiO₂ systems, it hasbeen found that a narrow range of compositions of the Na₂ O--Ta₂ O₅--SiO₂ system have good glass-forming properties when the three soleessential ingredients are within the following ranges:

    ______________________________________                                        Ingredient         Mole Percent                                               ______________________________________                                               SiO.sub.2   37-55                                                             Ta.sub.2 O.sub.5                                                                          23-35                                                             Na.sub.2 O  20-33                                                      ______________________________________                                    

Glasses thus formed have a high index of refraction, and can bethermally in situ crystallized to mechanically strong, transparentglass-ceramics having a still higher index of refraction.

While the most stable glasses are obtained when the ratio of Na₂ O toTa₂ O₅ is 1 or slightly less than 1, it has been found that good glassesare also formed when the ratio of Na₂ O to Ta₂ O₅ is from about 0.7:1 upto 1.2:1. When the Ta₂ O₅ is present in a molar amount greater than theNa.sub. 2 O, the glass-forming tendency of the composition is increased.

Several compositions were prepared by melting together silica, Ta₂ O₅and Na₂ O in the mole percents set forth in the following table:

    ______________________________________                                        Mole Percent                                                                          Compositions                                                          Ingredient                                                                              A      B       C    D    E    F    G                                ______________________________________                                        SiO.sub.2 40     45      50   35   40   30   30                               Ta.sub.2 O.sub.5                                                                        30     27.5    25   30   20   25   35                               Na.sub.2 O                                                                              30     27.5    25   35   40   45   35                               ______________________________________                                    

Each of the compositions was heated to a temperature sufficient to meltthe ingredients and then held at that temperature for about 1 - 24hours, the time being dependent on the specific composition and how longit took for the melting to be completed and a homogeneous melt to beformed. The melt was periodically stirred during this time. Each moltencomposition was then poured onto a metal plate and another metal plateimmediately placed over it to quench the composition. The glass-formingtendencies of each composition were then judged on the ability of themelt to be quenched into glass chips between the metal plates.

Compositions A, B, and C, which come within the composition range of theinvention, formed glasses readily, while compositions E, F, and G havingat least one ingredient outside the composition range of the invention,did not form any glass whatsoever. Composition D, which was just outsidethe composition range, had very poor glass-forming qualities, andconsequently the quenched plates had only small areas of glass.

As seen in FIG. 1 of the drawings, Compositions A, B, and C come wellwithin the glass formation area, while Composition D is on theborderline, and Compositions E, F, and G are well outside such area.When each of Compositions A - C was subjected to temperatures sufficientto first nucleate and then crystallize the glass, very good transparentglass-ceramics were obtained from the glass Compositions A, B, and C. Asseen in FIG. 2 of the drawing, Composition D falls outside of thetransparent glass-ceramic area as, of course, do Compositions E, F, andG. Glasses could not be formed from these latter three compositions, andComposition D did not produce a glass which could be heat treated toform a glass-ceramic.

The following table sets forth the heat treatment schedules utilized inthermally in situ crystallizing the glasses of the invention toglass-ceramics, and also shows the indices of refraction of the glassesand glass-ceramics.

                  TABLE II                                                        ______________________________________                                        Properties                                                                     and           Composi-  Composi-  Composi-                                   Treatment      tion A    tion B    tion C                                     ______________________________________                                        Index of Refrac-                                                              tion (Glass)   1.825     1.817     1.804                                      Heat Treatment 1570(16)  1625(2)   1625(64)                                   Temp. °F                                                                              1675(1)   1750(2)   1800(1/4)                                  (Time, hours)  1725(1/4)                                                      Index of Refraction                                                           (Glass-Ceramic)                                                                              1.895     1.884     1.858                                      ______________________________________                                    

NaTaO₃ was identified as the predominant crystal phase in theglass-ceramics obtained from the three compositions given above.

As evident in the above Table II, the index of refraction of thecrystallizable glass is quite high and it has been found that glasses ofthe invention can be prepared having indices of refraction within therange of about 1.80 to 1.95. When the glasses are subjected to thecrystallization schedule, the resultant glass-ceramics have a higherindex of refraction, and preferably within the range of about 1.83 to2.00.

The indices of refraction of the glasses and glass-ceramics of the aboveinvention are much higher than comparable indices of refraction of SiO₂--Na₂ O glasses which have indices within the range of about 1.48 to1.52. If CaO or BaO is incorporated in the SiO₂ --Na₂ O glasses, theindices of refraction come within the ranges of about 1.5 - 1.56 andabout 1.5 - 1.59 respectively. See The Properties of Glass by George W.Morey, Second Edition, 1954, published by Reinhold PublishingCorporation, New York, and particularly pages 378-382 thereof.

It is unexpected to find that thermally crystallizable glasses can beformed from the Na₂ O--Ta₂ O₅ --SiO₂ system wherein each of the threesole essential ingredients comes within a critical range and furthermorethat each of such glasses has an index of refraction which is relativelyhigher than one would expect.

Also within the broad spectrum of the alkali-Ta₂ O₅ --SiO₂ systems, ithas been found that certain compositions of the Na₂ O--Li₂ O--Ta₂ O₅--SiO₂ system coming within a narrow critical range also have goodglass-forming properties, provided that each of the four essentialingredients is present within the following ranges:

    ______________________________________                                        Ingredient          Mole Percent                                              ______________________________________                                         SiO.sub.2          27-45                                                      Ta.sub.2 O.sub.5   30-45                                                      Li.sub.2 O + Na.sub.2 O                                                                          20-35                                                     ______________________________________                                    

the molar ratio of Li₂ O to said Na₂ O is within the range of from 0.8:1to 1.2:1 and the molar ratio of Na₂ O + Li₂ O to Ta₂ O₅ is within therange of 0.5:1 to about 1:1.

To show the criticality of having the glass-forming compositions comewithin the above ranges, a number of compositions were prepared bymelting together silica, Ta₂ O₅, Li₂ O and Na₂ O for a period of from 1to 24 hours with constant stirring. Each of the compositions was thenquenched between metal plates as discussed above with respect to the Na₂O--Ta₂ O₅ --SiO₂ compositions. The following compositions were prepared:

                                      TABLE III                                   __________________________________________________________________________    Mole Percent                                                                  Ingred-                                                                            Compositions                                                             ients                                                                              H   I   J   K  L   M   N   O P  Q                                        __________________________________________________________________________    SiO.sub.2                                                                          40  30  30  30 35  35  40  20                                                                              30 30                                       Ta.sub.2 O.sub.5                                                                   30  35  40  45 35  40  40  40                                                                              25 50                                       Na.sub.2 O                                                                         15  17.5                                                                              15  12.5                                                                             15  12.5                                                                              10  20                                                                              22.5                                                                             10                                       Li.sub.2 O                                                                         15  17.5                                                                              15  12.5                                                                             15  12.5                                                                              10  20                                                                              22.5                                                                             10                                       n.sub.D                                                                            1.875                                                                             1.920                                                                             1.955  1.914                                                                             1.945                                                                             1.934                                             __________________________________________________________________________

Glasses were formed from Compositions H through N inclusive, all ofwhich come within the aforementioned critical range, while CompositionsO, P and Q, falling outside the critical composition range, did not formany glasses whatever. As may be seen in FIG. 3 of the drawing,Compositions H through N inclusive fall within the glass-formation area,while Compositions O, P and Q are outside of the defined area.

Of the glasses of Compositions H through N inclusive, all were heattreated to form transparent glass-ceramics with the exception of theglass of Composition H. As seen in FIG. 4 of the accompanying drawing,Composition H falls outside of the transparent glass-ceramic area. Thus,while the glasses of this embodiment of the invention come within thecritical range set forth above, thermally crystallizable glasses andglass-ceramics made therefrom come within the following critical range:

    ______________________________________                                        Ingredient          Mole Percent                                              ______________________________________                                         SiO.sub.2          27-45                                                      Ta.sub.2 O.sub.5   32-45                                                      Li.sub.2 O + Na.sub.2 O                                                                          20-35                                                     ______________________________________                                    

the molar ratio of Li₂ O to said Na₂ O to is within the range of from0.8:1 to 1.2:1 and the molar ratio of Na₂ O + Li₂ to Ta₂ O₅ is withinthe range of 0.5:1 to about 1:1.

present when the Ta₂ O₅ is less than 32 mole percent (it was prsent inan amount of 30 mole percent in Composition H), the glass is not athermally crystallizable glass and a glass-ceramic cannot be formedtherefrom.

Glass compositions I, J, K, and L were subjected to the heat treatmentschedule set forth in the following table whereby each of the glasseswas thermally in situ crystallized to a transparent glass-ceramic.Various properties of the glasses and glass-ceramics are set forth inthe following table:

                                      TABLE IV                                    __________________________________________________________________________                 Compositions                                                                  I     J     J     J     K     L     L                            Heat Treatment                                                                             1550(2)                                                                             1475(2)                                                                             1475(2)                                                                             1400(2)                                                                             1625(2)                                                                             1470(2)                                                                             1500(3)                      Temp. °F(Time, Hrs.)                                                                1675(1/2)                                                                           1650(1/2)                                                                           1600(1/4)                                                                           1950(1/4)                                                                           1750(1/4)                                                                           1500(1)                            Index of Refraction                                                            Glass       1.920 1.955 1.955 1.955       1.914 1.914                         Glass-Ceramic                                                                             2.000 2.015             2.022 1.973 1.967                        Crystal Phase                                                                              NaTaO.sub.3                                                                         NaTaO.sub.3       NaTaO.sub.3                                                                         NaTaO.sub.3                        Dielectric Constant                                                           (Room temp., 0.5 MHz)    62.5  85.5              66.3                         Dielectric Constant                                                           (Room temp., 1.0 MHz)                                                                            99                                                         Dissipation Fractor %                                                         (Room temp., 0.5 MHz)    0.8   0.8               0.68                         Dissipation Factor %                                                          (Room temp., 1.0 MHz)                                                                            0.75                                                       Temp. Coefficient of                                                          Capacitance                                                                   (-50°C. to 100°C)                                                                        0                                                    __________________________________________________________________________

NaTaO₃ was identified as a predominant crystal phase in glass-ceramicsformed from the compositions given above. LiTaO₃ and an unidentifiedphase are sometimes present as another crystal phase in theglass-ceramics.

Again, as is evident from the above table, the index of refraction ofthe crystallizable glass is quite high, coming within the range of1.85 - 2.0. When crystallized to a glass-ceramic, the index is evenhigher and preferably comes within the range of about 1.90 to 2.10.

Glass-ceramics of the invention coming within the Na₂ O--Li₂ O--Ta₂ O₅--SiO₂ system also have high dielectric constants of at least 50 andpreferably as high as 100 or more. Moreover, such glass-ceramics have adielectric loss or dissipation factor of less than 3% and preferablyless than 1%.

While each of the glasses and glass-ceramics of this system has a veryhigh index of refraction, the index of each glass-ceramic can be variedto a desired figure by varying the heat-treatment schedule whenthermally in situ crystallizing the glasses. The amount of increase inthe index is dependent upon the heat treatment. This can be seen fromthe following table wherein the glass of Composition J referred to abovewas subjected to different heat-treating schedules, so that the index ofrefraction in the final product differed, depending upon the temperatureand the times. The heat treatment and the resulting indices ofrefraction imparted to the resulting glass-ceramics are shown in thefollowing table:

                  TABLE V                                                         ______________________________________                                        Glasses of Composition J                                                                 No. 1  No. 2    No. 3    No. 4                                     Heat Treatment                                                                Temp. °F, (Time,hrs)                                                                1425(2)  1475(2)  1475(2)                                                                              1475(2)                                                                1500(1/2)                                                                            1650(1/2)                               Index of Refraction                                                            Glass       1.955    1.955    1.955  1.955                                    Glass-Ceramic                                                                             1.962    2.004    2.005  2.015                                   ______________________________________                                    

These high indices of refraction for the glasses and glass-ceramics,exemplified in Tables IV and V, are considerably higher than the indicesof refraction of the Li₂ O--Na₂ O--SiO₂ glasses which fall within therange of about 1.5 to 1.54 according to Morey, The Properties of Glass,supra.

While the glasses and glass-ceramics of the invention have beendiscussed in terms of the sole essential ingredients, it will be evidentto those skilled in the art that minor amounts preferably not more than10% and most preferably no more than 5% by weight of other metal oxideswhich are compatible with the glass and glass-ceramic compositions canbe used. Care should be taken, however, to insure that such other metaloxides do not materially affect the basic characteristics of the glassesand glass-ceramics of the present invention, including high indices ofrefraction, high dielectric constants, low dielectric losses, and highrelative strengths.

Furthermore, while the glass-ceramics of the invention have beendisclosed as being transparent, so that they can be used in many deviceswhere transparency is important, it will be appreciated that if only thehigh dielectric constant of the glass-ceramic together with itsrelatively high strength is required, the glass-ceramic can be furtherheat-treated to form an opaque glass-ceramic.

Transparent glass-ceramics having two or more indices of refraction canbe prepared by subjecting different portions of the thermallycrystallizable glass to the particular different heating schedules whichproduce the desired refraction indices. Thus, a crystallizable glassarticle of Composition J could have one portion subjected to heatschedule No. 1 of Table V and another portion subjected to heat scheduleNo. 4 of the same Table. The resulting transparent glass-ceramic articlewould have an index of refraction of 1.962 along the first portion andan index of 2.015 along the second portion.

Lenses, including eyeglass lenses, which are normally convex at onesurface, can be made from flat sheets of transparent glass-ceramicwherein the center of the sheet or lens has a certain index ofrefraction and a number of concentric areas around the center are givendifferent indices of refraction by subjecting such areas to therequisite heat schedules so that the indices of refraction are inascending or descending magnitudes from the center to the outerperiphery of the lens. Thus, using Composition J and the heat treatmentsset forth in Table V, the center of the transparent glass-ceramic lensof this composition J can have an index of refraction of 2.015 and thethree concentric areas around the center can have indices of refractionof 2.005, 2,004, and 1.962, respectively. By utilizing an appropriatetemperature gradient from the center to the edge, a smooth andcontinuous variation of the index of refraction from the center to theedge can be achieved. Changing the temperature gradient will change theresulting index of refraction gradient. This is an advantage over theFresnel type lens where discrete changes in the index are achieved bygluing together glasses having different indices. Means for applying thegradient temperature to the thermally crystallizable glass are known inthe art. Such lenses can be made to produce the same refraction of lightrays as presently obtained by grinding the surface of a piece of glassto shape a particular lens. By combining grinding and the index ofrefraction gradient, even greater refraction of light can be obtainedthan by utilizing either the grinding or the index gradient alone. Theprocess of imparting concentric indices of refraction to a thermallycrystallizable glass to form a glass-ceramic article can be applied to anumber of such glass articles, including transparent telescope mirrorblanks which are made of glass-ceramic.

Another important application of this embodiment of the invention is inoptical circuitry. By heat-treating a glass block or article so as toform a path of any desired shape and configuration within said block,with the glass-ceramic defining such a path having a particular higherindex of refraction than the surrounding material, light can be made tofollow such a path and can be bent as it travels about such a path.Thus, optical printed circuits can be readily made.

For example, a laser beam can be used to heat treat a particular spotwithin a thermally crystallizable glass block or chip and cause thatportion of the block to form a glass-ceramic having a particular indexof refraction. As the laser beam moves along the block, a narrow pathdefined by a glass-ceramic having a particular index of refraction isformed, since the heat of the laser beam is localized. The index ishigher than that of the surrounding glass and light moving along thepath stays within the path.

While glasses of the present invention, namely, those within theparticular Na₂ O--Ta₂ O₅ --SiO₂ and Na₂ O--Li₂ O--Ta₂ O₅ --SiO₂ systemsare suitable for purposes of forming transparent glass-ceramics having aplurality of indices of refraction, other thermally crystallizableglasses can be used including those within the Na₂ O--K₂ O--Nb₂ O₅--SiO₂ system, K₂ O--Nb₂ O₅ --SiO₂ system (Na₂ O, Li₂ O, K₂ O)--Nb₂ O₅--Ta₂ O₅ --GeO₂ systems and SiO₂ --Al₂ O₃ --Li₂ O systems. An example ofa thermally crystallizable glass coming within the latter system is asfollows, with amounts expressed in weight percent:

    ______________________________________                                               SiO.sub.2                                                                            66.9                                                                   Al.sub.2 O.sub.3                                                                     20.9                                                                   Li.sub.2 O                                                                           3.9                                                                    TiO.sub.2                                                                            1.8                                                                    ZrO.sub.2                                                                            2.0                                                                    CaO    3.5                                                                    Na.sub.2 O                                                                           0.7                                                                    Sb.sub.2 O.sub.3                                                                     0.3                                                             ______________________________________                                    

Many glasses coming within the latter system and forming transparentglass-ceramics are known in the prior art.

An example of the glasses coming within the K₂ O--Nb₂ O₅ --SiO₂ systemsuitable for forming glass-ceramics having an index of refractiongradient is as follows:

    ______________________________________                                               Ingredient                                                                            Mole Percent                                                   ______________________________________                                               SiO.sub.2                                                                             55                                                                    Nb.sub.2 O.sub.5                                                                      22.5                                                                  K.sub.2 O                                                                             22.5                                                           ______________________________________                                    

The batch ingredients for a 5 kg melt were melted in a crucible at atemperature of 2200°F for 24 hours, quenched in water and then remeltedand quenched several times. The last remelt was held at the temperatureof 2200°F for 5 hours. The crucible containing the molten glass was thenplaced within an insulated fire brick which had been previously cut toprovide an opening at the bottom thereof to permit the underside of thecrucible to extend therethrough. The fire brick and crucible were thenplaced on a 1-inch thick steel plate which had been preheated to atemperature of 900°F with the fire brick being supported on 1/8 inchthick blocks and the undersurface of the crucible in contact with thesteel plate. The top of the crucible was covered with quartz felt andthe molten glass was permitted to cool to 1200°F. The steel plate, firebrick and crucible were then placed in an annealing furnace having atemperature of 1180°F for about a half hour, at the end of which timethe furnace was turned off and the glass permitted to cool to ambienttemperature at furnace rate.

The annular transparent glass ceramic which had formed had a radialindex of refraction gradient. While the index of refraction was 1.767,it varied in a regular manner from the center to the edge with adifference in th index of -.002. Very good radial symmetry was observedwith respect to the index of refraction gradient.

Having fully described the invention, what is claimed is:
 1. A thermallycrystallizable glass consisting essentially of 37-55 mole percent SiO₂,23-35 mole percent Ta₂ O₅ and 20-33 mole percent Na₂ O.
 2. The thermallycrystallizable glass as defined in claim 1 wherein the molar ratio ofNa₂ O to Ta₂ O₅ is about
 1. 3. The thermally crystallizable glass asdefined in claim 1 wherein the molar ratio of Na₂ O to Ta₂ O₅ is withinthe range of about 0.7:1 to about 1.2:1.
 4. The thermally crystallizableglass as defined in claim 1 wherein said glass has an index ofrefraction of at least 1.78.
 5. The thermally crystallizable glass asdefined in claim 4 wherein said glass has an index of refraction withinthe range of about 1.80 to 1.95.
 6. A glass having an index ofrefraction of at least 1.9 and consisting essentially of about 27-45mole percent SiO₂, 30-45 mole percent Ta₂ O₅ and 20-35 mole percent ofLi₂ O + Na₂ O wherein the molar ratio of said Li₂ O to said Na₂ O iswithin the range of from 0.8:1 to 1.2:1 and the molar ratio of Na₂ O +Li₂ O to Ta₂ O₅ is within the range of 0.5:1 to about 1:1.
 7. Athermally crystallizable glass consisting essentially of about 27-45mole percent SiO₂, 32-45 mole percent Ta₂ O₅ and 20-35 mole percent ofLi₂ O plus Na₂ O, wherein the molar ratio of said Li₂ O to said Na.sub.2 O is within the range of from 0.8:1 to 1.2:1 and the molar ratio ofNa₂ O + Li₂ O to Ta₂ O₅ is within the range of 0.5:1 to about 1:1. 8.The thermally crystallizable glass as defined in claim 7 wherein themolar ratio of Li₂ O to Na₂ O is about 1 and the molar ratio of Na₂ O +Li₂ O to Ta₂ O₅ is about .7.
 9. The thermally crystallizable glass asdefined in claim 8 wherein said glass has an index of refraction of atleast 1.9.
 10. The thermally crystallizable glass as defined in claim 9wherein said glass has an index of refraction within the range of 1.9 to2.10.